Context Previous studies have shown oseltamivir, a neuraminidase inhibitor,
to be effective in preventing influenza and treating experimental influenza.

Objective To evaluate the efficacy and safety of oseltamivir in the treatment
of naturally acquired influenza infection.

Design Randomized, placebo-controlled, double-blind study conducted January
through March 1998.

Setting Sixty primary care and university health centers throughout the United
States.

Participants A total of 629 healthy nonimmunized adults aged 18 to 65 years with
febrile respiratory illness of no more than 36 hours' duration with temperature
of 38°C or more plus at least 1 respiratory symptom and 1 constitutional
symptom.

Conclusions Our data suggest that oral oseltamivir treatment reduces the duration
and severity of acute influenza in healthy adults and may decrease the incidence
of secondary complications.

Influenza is estimated to be responsible for from 13.8 to 16.0 million
excess respiratory illnesses per year in the United States among individuals
younger than age 20 years, and for 4.1 to 4.5 million excess illnesses in
individuals aged 20 years and older.1 The symptoms
of this disease are debilitating, and a typical case of influenza is associated
with approximately 5 to 6 days of restricted activity, 3 to 4 days of bed
disability, and about 3 days lost from work or school.2,3
In addition, complications of acute influenza, including bronchitis and pneumonia,
are common, with approximately 150,000 hospitalizations and 20,000 deaths
from influenza-related complications yearly in the United States.4

Although an effective vaccine is available, current treatment options
for influenza are limited. Amantadine hydrochloride and rimantadine hydrochloride
are effective for early treatment of influenza A, but have no activity against
influenza B viruses and are associated with the emergence of resistant viruses
in treated individuals.5 In addition, both
drugs can cause central nervous system and gastrointestinal adverse effects,
which may be more common in older individuals.6
The neuraminidase inhibitor zanamivir is also effective in the treatment of
influenza.7-9 However,
zanamivir must be administered topically (ie, by inhalation or intranasally)
or parenterally10 to be effective.

Oseltamivir carboxylate is a potent and specific influenza neuraminidase
inhibitor11 that inhibits replication of a
wide variety of influenza A and B viruses in vitro.12
Oseltamivir phosphate (oseltamivir) is the ethyl ester prodrug of oseltamivir
carboxylate, and oral administration of this agent results in sustained plasma
levels of the active drug.13 Oseltamivir was
effective when administered orally in treatment of influenza A infection in
animals12 and experimentally infected humans14 and in prevention of influenza illness.15

We designed the current trial to test the hypothesis that early treatment
with oseltamivir would be well tolerated and result in reductions in the severity
and duration of acute naturally acquired influenza illness. The study was
conducted to determine safety and clinical efficacy of 2 different doses of
oseltamivir administered for 5 days beginning within 36 hours of symptom onset
in adults with microbiologically proven influenza.

Methods

This study was in full conformance with the principles of the Declaration
of Helsinki.16 Institutional review boards
from each participating center reviewed and approved the protocol and consent
form prior to study start. All participants gave written informed consent
prior to enrollment and were financially compensated for their participation.

Clinical Study Design

The study was conducted as a double-blind, stratified, randomized, placebo-controlled,
multicenter trial conducted during the influenza epidemic season from January
to March 1998 at 60 centers in the United States.

Participants

Previously healthy adults aged 18 to 65 years who presented within 36
hours of onset of influenza symptoms and who had documented oral temperature
of 38°C or higher at enrollment plus 1 or more respiratory symptom (cough,
sore throat, or nasal symptoms) and 1 or more constitutional symptom (headache,
malaise, myalgia, sweats and/or chills, or fatigue) were enrolled. Women were
required to have a negative urine pregnancy test before drug administration.

Individuals were excluded from the study if they had received influenza
vaccination in the 12 months prior to the beginning of the study; had active,
clinically significant chronic illness or human immunodeficiency virus disease;
were receiving systemic steroids or other immunosuppressants; or had a history
of alcohol or drug abuse.

Drug Administration

Participants were randomly assigned to 1 of 3 treatment groups: oseltamivir,
75 mg or 150 mg orally twice daily, or matching placebo for 5 days. Randomization
occurred at the time of study entry by telephone contact with an automated
service that had sole access to the code key and was stratified by study site
and smoking behavior. Participants and staff remained blinded to allocation
status throughout the study. Compliance was assessed by checking patient records
of the date and time of each dose and verified by counting capsule returns
for each patient.

Participants were instructed to use acetaminophen, provided on enrollment
by study personnel, for symptom relief. The use of acetaminophen and any other
medications for symptom relief was recorded on the case record form.

Clinical Monitoring

Participants recorded the severity of 7 influenza symptoms (cough, nasal
obstruction, sore throat, fatigue, headache, myalgia, and feverishness) using
a 4-point scale (0, absent; 3, severe) twice daily for 21 days. Oral temperature
was also taken by the patient with a digital thermometer twice daily and recorded
on the diary card. On each day during the dosing period, participants recorded
their ability to perform usual activities on a diary card using an 11-point
visual analog scale (unable to perform normal activity, 0; fully able to perform
normal activity, 10). In addition, participants were asked to complete a visual
analog scale of their opinion of overall health status on which they were
requested to provide an assessment of their normal preinfluenza health on
an 11-point scale (0, worst health and 10, best possible health). Following
this, they were asked to record their assessment of health status at baseline
and over a 24-hour period once daily in the evening. The use of these scales
had been validated in a pilot study conducted among English-speaking volunteers
during the influenza season in Australia in 1997. The scales were demonstrated
to be easily comprehended by English-speaking volunteers and correlate well
with other questions about activity and with influenza symptom scores (Influenza
Questionnaire Pilot Study Report, August 1997, Hoffmann-La Roche, data on
file).

Laboratory Methods

Anterior nose and posterior pharyngeal throat swabs for isolation of
influenza virus were taken at baseline (day 0) and on days 1, 3, 5, and 7
of the study. Swabs were taken from both nostrils and the throat, placed into
3 mL of viral transport medium, and transported on wet ice to a central laboratory
within 24 hours. At the central laboratory, the swab samples were eluted,
divided into aliquots, and immediately frozen at −70°C. Initial
virus isolation was performed in primary rhesus monkey kidney cells, and all
samples for an individual were tested in the same assay run. The presence
of virus in the cell culture was determined by immunoflorescence, using a
pool of antibodies specific for influenza A and B viruses. Type and subtype
of the influenza viruses isolated were determined by hemagglutination inhibition
assay (HAI) using specific antisera. The titer of virus in virus-positive
samples was determined by serial dilution of a fresh aliquot in tertiary cynomolgus
monkey kidney cells. Titers were calculated as log10 tissue culture
infective dose50 (TCID50)/mL of viral transport medium
using the Spearman Karber equation.17 The last
isolate recovered from each participant was tested for neuraminidase susceptibility
to oseltamivir carboxylate by fluorometric substrate assay11
(total of 175 samples tested).

Serum samples for HAI antibody titer were obtained at baseline and on
day 21 after enrollment. The HAI assays were performed by standard methods
using antigens known to be circulating during the 1997-1998 season (influenza
A/Shenzhen/95 [H1N1], A/Wuhan/95 [H3N2], and A/Sydney/97 [H3N2], and B/Harbin/95).18 The definition of seroresponse was a 4-fold or greater
rise in type-specific antibody between the baseline and day 21 samples. Primary
virus isolation and serum HAI antibody testing were performed at Viromed Laboratories
Inc, Minneapolis, Minn, and virus titrations were performed at Erasmus University,
Rotterdam, the Netherlands. All laboratory tests were performed by individuals
blinded to study assignment.

Case Definition

For the primary efficacy analysis, laboratory-documented influenza infection
was defined as isolation of influenza virus from nasal secretions and/or a
4-fold or greater HAI antibody response.

Efficacy End Points

The primary efficacy end point was time to resolution of illness, defined
as time from study drug initiation to time of alleviation of symptoms, among
individuals with influenza infection. Symptom alleviation was considered to
occur at the start of the first 24-hour period in which all influenza symptoms
were scored 1 or less (mild or none) and remained so for 24 hours. The effect
of treatment on the severity of illness was also assessed by an area under
the curve (AUC) analysis of total symptom scores. For this analysis, the AUC
was calculated as the product of the daily symptom score times the duration
of illness, as defined above, and expressed as "score-hours." Other end points
included duration and severity of individual symptoms; incidence of secondary
complications of influenza such as otitis, bronchitis, sinusitis, and pneumonia;
and quantity of viral shedding.

Quality-of-life measures included time to return to normal states of
health and activity. Return to normal status was defined as the time (in hours)
from study drug initiation to the first 24-hour period in which participants
returned to their normal state and remained so for 24 hours.

Data Analysis

The primary efficacy analyses were carried out for participants who
received at least 1 dose of study drug and had laboratory-confirmed influenza
infection. A secondary efficacy analysis was also performed for all subjects
who received study drug irrespective of laboratory evidence of infection.
For the primary end point, comparisons were performed using a weighted Mantel-Haenszel
test stratified for region and smoking status. Investigation of the consistency
of treatment effects between regions and smoking groups was performed. Subjects
who withdrew before symptoms were alleviated were censored at the time of
withdrawal.

For extent and severity of illness, the placebo group was compared with
each active treatment group using an extended Wilcoxon rank sum test (van
Elteren) stratified for region and smoking status. Duration of viral shedding
and time to return to normal states of health and activity were compared between
placebo and each active treatment group using a weighted Mantel-Haenszel test.
Viral titers were assessed by the extended Wilcoxon rank sum test (van Elteren).
Adjustments of P values for multiple comparisons
of treatment groups were made for the primary end point, but not for other
parameters. The within-group 95% confidence intervals (CIs) for medians were
calculated using the method of Brookmeyer and Crowley.19

The safety population included all participants who received at least
1 dose of drug and for whom postbaseline safety data were available. Formal
statistical hypothesis testing was not performed; 95% CIs for the difference
in nausea and vomiting rates were calculated using a normal approximation.

Sample size calculations assumed an overall 2-sided 5% significance
level to be distributed equally between the comparisons of 75 mg and 100 mg
of oseltamivir, respectively, vs placebo. A sample size of 85 in each group
was estimated to have at least 80% power to detect a difference of 1.5 days
in time to alleviation of all symptoms, assuming a common SD of 3 days and
a significance level of 0.025. Sample size calculations were performed using
a normal approximation to the Wilcoxon rank sum test. All analyses were performed
using SAS software, version 6.12 (SAS Institute, Cary, NC).

Results

A total of 629 participants were recruited from 60 centers. The disposition
of the participants in the trial is shown in Figure 1. Of the 629 individuals enrolled in the study, 209, 211,
and 209 were randomized to receive placebo; oseltamivir, 75 mg; and oseltamivir,
150 mg, respectively. Two participants (1 in the 75-mg group and 1 in the
150-mg group) withdrew from the study after randomization but before the study
drug was dispensed. In addition, a small number of participants did not receive
treatment as randomized because of dispensing errors. One person randomized
to receive placebo received 75 mg and 1 person, 150 mg of oseltamivir twice
daily; 1 person randomized to oseltamivir, 75 mg twice daily, received placebo,
and 1 received 150 mg of oseltamivir twice daily; and 1 person randomized
to receive oseltamivir, 150 mg twice daily, received 75 mg twice daily. For
purposes of efficacy analysis, participants were analyzed according to the
group to which they were initially randomized excluding the 2 subjects who
withdrew before medication was dispensed; for purposes of the safety analysis,
participants were analyzed according to the drug actually received. In addition,
safety data were not available from 4 participants in each group and were
therefore not included in the safety population. Of the 627 participants who
received treatment 374 (59.6%) were influenza infected and included in the
primary efficacy analysis. Of these, 343 (92%) had influenza A (H3N2) and
299 (80%) were culture positive. The demographic and clinical characteristics
of the participants and the proportions infected with influenza virus in each
group are described in Table 1.
No important differences were observed among treatment groups. Median duration
of illness prior to enrollment and average symptom score and mean temperature
on enrollment were similar in all 3 infected treatment groups. Adherence to
the assigned treatment regimen was excellent. Overall, 97% of placebo recipients,
98% of 75-mg oseltamivir recipients, and 95% of 150-mg oseltamivir recipients
who completed the study took all 10 tablets as directed.

Clinical Outcomes

The effect of oseltamivir treatment on alleviation of illness is shown
in Table 2. Both dose levels of
oseltamivir resulted in statistically significant reductions in the duration
and severity of illness among those infected with influenza virus. The duration
of illness, defined as the time to the beginning of the first 24-hour period
in which all influenza symptoms were rated as mild or less, was 103.3 hours
(4.3 days) in the placebo group. In contrast, the duration of illness was
reduced to 71.5 hours (3.0 days) in the 75-mg group, and to 69.9 hours (2.9
days) in the 150-mg group. Similarly, treatment with oseltamivir at either
75 or 150 mg twice daily resulted in statistically significant reductions
(P<.001 and P = .006,
respectively) in the symptom score AUC, reflecting both the severity and duration
of illness. There were no differences between the 2 doses of oseltamivir with
regard to these effects. Overall, oseltamivir treatment reduced median duration
of illness by more than 30% (P = .006) and median
severity of illness by approximately 40% (P<.001).
Treatment benefit was apparent soon after administration, with individuals
receiving oseltamivir reporting alleviation of illness more frequently than
those receiving placebo as early as 24 hours after the onset of treatment
(Figure 2). In addition, individuals
receiving oseltamivir reported significantly more rapid return to normal overall
health (24-46 hours faster) and resumption of usual activities (45-68 hours
faster) (Table 2).

Duration and severity of each individual influenza symptom recorded
in infected subjects were also reduced with oseltamivir (Table 3). In particular, duration of cough was reduced from a median
of 55 hours in the placebo group to 31 hours (43% reduction) in the 75-mg
group, and 40 hours (27% reduction) in the 150-mg group. The duration of myalgias
was also reduced, from a median of 28 hours in placebo recipients to 16 hours
(42% reduction) in the 75-mg group, and 19 hours (32% reduction) in the 150-mg
group.

The daily proportion of infected subjects reporting fever (oral temperature
of ≥38°C) was reduced by treatment, and a reduction in fever was evident
within 24 hours of therapy (Figure 3).
The percentage of subjects with fever at 24 hours was 39% in the placebo group,
compared with 26% in the 75-mg group (13% difference; 95% CI, 25%-2%) and
21% in the 150-mg group (18% difference; 95% CI, 29%-6%). The use of acetaminophen
also was lower in individuals receiving oseltamivir (median, 4.0-4.5 g per
patient) than in those receiving placebo (median, 5.5 g per patient).

Overall incidence of physician-diagnosed secondary complications (predefined
as pneumonia, bronchitis, sinusitis, and otitis media) in those with influenza
was reduced by 50% in individuals receiving oseltamivir vs those receiving
placebo. A similar reduction was observed in the proportion of these individuals
receiving antibiotics for influenza complications (Table 4).

Because it is likely that an antiviral drug to treat influenza would
be used in the absence of a specific microbiologic diagnosis, we also performed
an analysis of the effect of treatment on all individuals who received medication
regardless of microbiologic results (Table
2). Oseltamivir also had a significant benefit in this analysis.
Median time to alleviation of symptoms was reduced by 21% and 23% and severity
of illness by 23% and 29% in recipients of 75 mg and 150 mg twice daily, respectively.
In addition, recipients of oseltamivir returned to normal health and activities
more rapidly than placebo recipients among the group as a whole. Complications
of influenza occurred in 28 placebo recipients (13%), 18 recipients (9%) of
75-mg oseltamivir, and 13 recipients (6%) of 150-mg oseltamivir (combined
oseltamivir results vs placebo for Fisher exact test, P = .02).

Virologic Outcomes

Analysis of the effect of oseltamivir on virus shedding was conducted
on the subset of participants with a baseline sample positive for influenza
and for whom appropriate samples were collected on days 1, 3, and 5 (Table 5). The median viral titer at enrollment
was similar, and a rapid decline in virus shedding was observed in all 3 groups.
After 24 hours of treatment, median viral titers had decreased by 1.2 logs
in the placebo group vs 1.7 and 2.0 logs in the 75-mg and 150-mg oseltamivir
groups, respectively, but these differences were not statistically significant.
The proportions of participants shedding virus at each time point were similar
in all 3 groups, and few participants in any group were shedding virus by
72 hours after initiation of therapy. There was no evidence of a rebound of
virus shedding at drug discontinuation.

All isolates from the last day of virus shedding were tested for drug
susceptibility, and a virus with altered neuraminidase susceptibility to oseltamivir
carboxylate was detected in 1 recipient of 75-mg oseltamivir on day 3 of treatment.
The 50% neuraminidase inhibitory concentration for oseltamivir carboxylate
for the pretreatment isolate from the subject was 0.93 nmol/L, while the posttreatment
50% neuraminidase inhibitory concentration was 6814 nmol/L. Sequence data
from this virus are not yet available. The individual from whom resistant
virus was isolated was a 49-year-old female smoker who was enrolled 20 hours
after onset of symptoms. She complained of moderate cough for 8 days, so that
the duration of illness in this individual was 8 days. None of her symptoms
became worse after cessation of therapy, and she did not experience an influenza
complication. Virus was not isolated from this individual on days 5 or 7.

Proportions of participants with 4-fold or greater rises in HAI antibody
were comparable among treatment groups (data not shown). Similarly, there
were no differences among treatment groups for the geometric mean HAI antibody
rise from baseline to convalescence (mean, 16.1-fold, 15.9-fold, and 15.7-fold
rise in those receiving placebo and oseltamivir, 75 mg and 150 mg, respectively).

Safety Analysis

Oral oseltamivir was well tolerated. No changes occurred in standard
laboratory safety evaluations in any treatment groups (data not shown) and
there were no drug-related serious adverse events. Upper gastrointestinal
effects (nausea or nausea with vomiting) were reported more frequently in
those receiving oseltamivir. For nausea, these rates were 7.4% (15/204) for
placebo recipients, 17% (35/206) for recipients of 75-mg oseltamivir and 19%
(39/205) for recipients of 150-mg oseltamivir (for overall difference in the
3 groups, P = .002; for differences between placebo
and 75-mg and 150-mg oseltamivir, P = .002 and P<.001, respectively). For vomiting, the rates were
3.4% (7/204) with placebo, 13.1% (27/206) with 75-mg oseltamivir, and 15.1%
(31/205) with 150-mg oseltamivir (P<.001 for overall
difference in the 3 groups and differences between placebo and 75-mg and 150-mg
oseltamivir). Despite mild gastrointestinal intolerance, the number of participants
withdrawing from the study during therapy was low (3% in the placebo group,
1.5% in the 75-mg group, and 2% in 150-mg group), and only 1 participant (oseltamivir,
150-mg group) withdrew prematurely because of gastrointestinal events.

Comment

The results of this study indicate that the oral neuraminidase inhibitor
oseltamivir is an effective treatment for acute influenza in adults. Treatment
within 36 hours of symptom onset resulted in approximately 30% reduction in
illness duration, approximately 40% in illness severity, and by more than
2 days in the time to resumption of usual activities vs placebo. Treatment
with oseltamivir resulted in reductions in fever and duration and severity
of individual influenza symptoms, including cough and myalgia, 2 of the more
disabling symptoms. A parallel trial conducted in Canada and Europe during
the same influenza season found similar benefits with oseltamivir treatment.20 These outcomes represent a clinically significant
reduction in overall disease burden with oseltamivir treatment.

Complications of influenza, such as bacterial superinfections, pneumonia,
and hospitalization, are common in individuals with certain chronic medical
conditions.21,22 Less severe complications,
such as bronchitis and sinusitis in healthy adults and otitis media in children,
have been reported at rates between 1% and 30% in various studies.23 In the current study, the frequency of secondary
complications of influenza and associated antibiotic use for presumed bacterial
complications were significantly reduced in oseltamivir-treated subjects.
These results are similar to those reported with zanamivir in studies conducted
in healthy adults or those with mild asthma.8,24
While no formalized criteria were used to define bronchitis, pneumonia, or
sinusitis, the functional definitions used in this study probably reflect
actual medical practice. Our findings raise the possibility that early treatment
of influenza with an effective antiviral drug might reduce the frequency of
complications in high-risk populations and potentially reduce hospitalization
rates.

Lost time from work or school and reductions in performance represent
important effects of influenza on healthy adults.25,26
While our study was not specifically designed to evaluate the economic benefit
of antiviral therapy, it is notable that the reduced levels of influenza symptoms
in treated subjects were accompanied by a significantly more rapid return
to normal activities, which may provide another rationale for early use of
antiviral treatment for influenza in previously healthy adults.

Comparisons of our study results with those of other antiviral approaches
to acute influenza are complicated both by differences between trials in the
specific outcome measurements and by the variable nature of influenza illness
each year. However, the levels of reductions in symptoms seen in this study
are broadly comparable to those described with acute treatment of influenza
with the M2 channel inhibitors amantadine and rimantadine27-32
and the topically applied neuraminidase inhibitor zanamivir.7-9
More rapid return to usual activities has also been reported following treatment
with rimantadine30,33 and zanamivir.7,9

Oseltamivir was generally well tolerated in this study, although nausea
and emesis occurred more frequently in treated individuals than in those receiving
placebo. Nausea resolved without treatment and despite continuation of drug
therapy and was not associated with an increase in discontinuation rates in
treated individuals compared with placebo recipients. Drug administration
with even small amounts of food appeared to prevent this effect.15
In pharmacokinetic studies, administration of drug with food was associated
with increased absorption,13 so that alleviation
of nausea by administration with food is consistent with gastric irritation
rather than a central drug effect. No other adverse effects or laboratory
value abnormalities were associated with oseltamivir treatment.

Virus replication in the lower respiratory tract could not be assessed
directly in the current study, but the reduced duration of cough and rate
of secondary bronchitis are consistent with preclinical studies showing that
oral administration delivers sustained levels of oseltamivir carboxylate throughout
the respiratory tract, including the sinus cavities and middle ear,34 and reduces lung virus titers in murine models.12,14 This suggests that oral administration
of oseltamivir results in antiviral effects in both the upper and lower respiratory
tract.

A potential advantage of neuraminidase inhibitors over existing M2 inhibitors
is their expanded spectrum of activity. In previous studies of zanamivir,
reductions in the duration of illness were seen with both influenza A and
B.7 Because of the low prevalence of influenza
B in the 1997-1998 winter season, only 9 individuals infected with influenza
B were enrolled in the study. However, results in these subjects were similar
to those of the group as a whole (data not shown).

Antiviral drug resistance has been one factor that has limited more
widespread use of amantadine and rimantadine.35
Resistant viruses emerge rapidly and relatively frequently in individuals
treated with these agents5,33
and can be transmitted to and cause disease in susceptible contacts.5,33,36 Antiviral resistance
to neuraminidase inhibitors has been observed much less frequently in human
studies to date, possibly because mutations in the conserved residues in the
neuraminidase can be associated with decreased replication fitness.37 One zanamivir-resistant virus with dual hemagglutinin
and neuraminidase mutations has been reported in an immunosuppressed child
receiving prolonged nebulized zanamivir for treatment of influenza B pneumonia38; the specific mutations responsible for resistance
in the virus isolated in our study have not yet been identified. However,
the results of our study also suggest that emergence of resistance during
short-term treatment of acute influenza in healthy adults may not be a clinically
significant problem for the neuraminidase inhibitors.

Several potential limitations of this study should be considered. A
substantial proportion of the 2120 individuals screened for enrollment did
not meet the entry criteria for the study, because they did not have typical
influenza symptoms, were not febrile, or had been ill for too long. Thus,
the results of the study should be considered in the context of appropriately
selective use of an antiviral drug for acute influenza. In addition, the study
protocol specifically excluded individuals with medical conditions that are
often associated with more severe influenza. However, assuming that the mechanism
by which influenza causes illness in these individuals remains the same, there
is little reason to think that the benefit of early treatment would not also
be seen in a higher-risk population. As expected, the greatest benefit of
therapy was seen in individuals with evidence of influenza virus infection.
However, analysis of the entire population also demonstrated a significant
benefit of treatment. In summary, our trial indicates that oseltamivir is
an effective treatment for acute influenza in healthy adults. The findings
provide a rationale for continued study of this agent for the treatment of
influenza, including studies in children and in high-risk populations.